In current building construction, particularly in commercial and institutional multistory buildings, the most common structural systems are cast-in-place concrete and steel framing with composite decking. Precast planks have been used in structural systems with either cast-in-place concrete topping slabs or a wider cast-in-place concrete joint to encase the steel beam top flanges for a composite structure. See, for example, U.S. Pat. No. 5,704,181 for slab to beam connection.
For cast-in-place concrete structures, the superstructure construction starts with placement of rebar and formwork for columns and walls, then erection of shoring system, followed by installation of floor formwork, placement of floor deck reinforcement, and concrete pour and finish. In order to reduce formwork cost, after the floor structure gains adequate strength, the formwork is removed and lifted to the level above and the floor structure is re-shored. The construction process repeats at each floor. Cast-in-place concrete structure requires massive labor and longer construction duration on site for all the construction activities. For steel superstructure, construction starts with erecting steel framing, then installing composite steel decks, welding shear connectors, laying reinforcement, and pouring concrete. Each floor deck requires extended time of preparation before concrete pour and followed by curing times after concrete pour. Both structural systems require long construction duration in the field, as further delays to project schedule may occur due to weather conditions.
In current construction practice, concrete is the most common material for floor slab construction due to its durability, fire resistance, and low cost. When concrete is cast, it is in plastic form and flowable, which leads to the floor deck being relatively flat and level at the beginning. To reduce project cost, it is the conventional practice to construct composite deck without shoring. After the deck in one bay has been cast, the concrete poured in the adjacent bay will cause the previous bay to deflect due to newly added concrete weight. For shored decks, either composite deck or concrete deck, after shoring is removed, floor decks will deflect under their own weight. Both shored and un-shored cast-in-place floor deck construction therefore result in uncontrollable deflection and irregular cracks. Floor levelness and flatness are crucial factors in floor finishing cost. Where floor levelness or flatness do not meet certain requirements, the floor deck has to be either filled up or grinded down before installing floor finishing materials, therefore, resulting in additional project costs and construction time.
There are various factors that can cause random cracks in concrete. Random cracks are typically due to concrete shrinkage and deflection or uneven settlement of support. Without treatment, random cracks may cause damage to the floor finishes and lead to associated repair costs. Ideally, if the cracks are located in a controlled manner, control joints can be placed in the floor finish to accommodate crack location and mitigate repair costs.
Typical concrete material is a mixture of cementitious material, sand, aggregate, water and admixture chemicals. The cementitious materials, such as Portland cement, react with water through a hydration process to produce a synthetic rock of higher strength. The amount of water not participating in the hydration process gradually moves through concrete by diffusion. For concrete slab on grade construction, a vapor barrier is placed under the slab to prevent vapor transmission from soil below; in the meantime, it blocks the concrete moisture from diffusing. Water behaves similarly in composite deck construction. The metal deck also stops moisture from moving downward. Both vapor barrier and metal deck slow down the concrete drying process. It may take years for concrete to dry out and result in costly moisture damage to floor finishing materials.
According to the U.S. Green Building Council, buildings use 40% of raw materials globally or 3 billion tons annually. Typical building structure life span is 50 years. At end of the building life, the common practice is to demolish, not deconstruct, the building. EPA estimates 170 million tons of building related construction and demolition debris was generated in the U.S. in 2003. Cast-in-place concrete structure, steel framing structure and precast plank structure cannot be re-used, though to some extent their materials can be recycled. Recycling, in contrast to re-using, incurs a large amount of energy to, for example, process old steel as scrap to produce new steel shapes. Most of the CO2 emission and energy consumption for building construction are from production of construction materials. To contribute to sustainability efforts, the construction industry should reduce raw material use, landfill, CO2 emission and energy consumption.
At the present time, there have been no structural systems nor construction processes that can adjust the floor levelness and control floor flatness, prevent irregular concrete cracks, require no cast-in-place concrete for slab on grade and floor deck, effectively control concrete moisture, and be deconstructed for reuse.